Planning ahead minimizes the risk of icing
Along with stumbling into thunderstorms, an inadvertent encounter with icing conditions ranks right near the top of a pilot's worst fears. And for good reason. Even small ice accretions can decrease an airfoil's lift, increase drag, and cause dangerous drops in airspeed. That's why the cardinal rule of thumb is to take evasive action fast at the very first sign of airframe icing. That goes for everything from the smallest single-engine airplane to the largest business jets. Having ice-protection systems, or flying an airplane certified for flight in known icing conditions, can buy you some time to make your escape, but know this: Many airplanes with full complements of ice protection equipment and known-icing certification have crashed after lingering too long in icing conditions.
How it happens
A common icing scenario goes like this: A pilot receives a weather briefing mentioning the chance of icing conditions, or even reported icing conditions, along his or her proposed route of flight, and launches anyway. Another scenario: A VFR-only, or even instrument-rated, pilot continues flying into deteriorating weather, eventually runs into full-blown instrument meteorological conditions, flies into clouds, and ices up.
In their chains of causality, then, icing-related accidents closely resemble one of the biggest killers in general aviation-continued VFR flight into instrument weather. The antidote to these accidents? Maintain better-than-VFR separation minimums from clouds and precipitation. A pilot who's flying 500 feet below one cloud deck or 1,000 feet above another in three miles' worth of visibility is probably a pilot who's flying in frontal or other adverse weather that can quickly turn to marginal VFR, then instrument weather. That's one thing if you're flying locally and are familiar with (plentiful) nearby airports. It's much more dangerous if you're flying a long cross-country over hostile terrain.
Types of icing
All pilots should know that there are two basic types of icing-clear and rime. Clear ice occurs most often in the 0 to minus 10 degree Celsius temperature range. As the name implies, clear ice is a clear coating over the airplane's leading edges. It's often found in cumulus clouds and unstable conditions.
Rime ice usually lurks in stratiform clouds with temperatures between minus 10 and minus 20 degrees Celsius. It has a milky, pebbly appearance, and first shows up as a thin white line on wing leading edges or other airframe protuberances, such as outside air temperature probes and antennas.
The icing process occurs when an airplane flies into clouds or precipitation composed of supercooled water droplets. Supercooled droplets are liquid, but at freezing temperatures. They remain liquid until an airplane flies into them. Then they quickly freeze on impact with the leading edges. Rime ice is usually slower to build than clear ice.
The worst of the worst
There are two additional types of icing that rank especially high on the danger scale. One, freezing rain (abbreviator: FZRA), is a fast-forming type of clear ice that occurs primarily in advance of winter warm fronts. It's caused by rain, snow, or ice crystals falling through a warmer layer of air at lower altitudes. Freezing rain was covered in last month's Wx Watch (see " Wx Watch: Fighting Freezing Rain," September Pilot). The big problem with freezing rain is that the very large droplets associated with this phenomenon run far back on airfoil surfaces and can quickly disrupt lift. The extra weight of the ice has a minimal effect on airplane performance.
But as bad as freezing rain is, there's another phenomenon that's orders of magnitude worse. That would be freezing drizzle (FZDZ). Freezing drizzle is characterized not just by large supercooled droplets, but also by its extremely high liquid water content. When freezing drizzle strikes an airplane, ice formations can become large and strangely shaped. Ridges of ice may form along the entire wingspan, causing aerodynamic havoc.
"Freezing drizzle can put you in dire straits in a couple of minutes," says William Rieke, NASA Glenn Center's chief of aircraft operations, and a veteran of many icing test flights. "It's much worse than freezing rain."
Freezing drizzle was studied heavily after the October 31, 1994, crash of an ATR-42 in Roselawn, Indiana. Re-searchers determined that supercooled "drizzle drops" likely caused ridges of ice to form aft of the deice boot-protected leading-edge surfaces. Official opinion is that the ridges blanketed the air flow over the ailerons, moved the center of lift aft on the wing chord, caused one wing to stall, and sent the ATR into a fatal spiral dive. Because the airplane was on autopilot, the crew couldn't tell that the ship's lift and aileron forces were changing at an alarming rate. When the autopilot, which was attempting to maintain level flight, automatically kicked off because of the excessive corrective forces it was applying, the surprised pilots could only watch the airplane roll over.
Freezing drizzle seems to occur most often in the Great Lakes and maritime regions, where the air in frontal systems can be loaded with huge amounts of liquid water. Results are pending from additional research, but the prevailing opinion these days is that freezing drizzle is predominantly a low-altitude phenomenon. That's bad news for pilots of piston-powered airplanes, because that's where most of their flying is done. The ATR's freezing drizzle encounter occurred between 10,000 and 8,000 feet msl, when it descended in a holding pattern.
Escape strategies
Viable strategies for escaping icing conditions depend on the conditions at hand. A descent to altitudes with warmer temperatures may solve the problem. A climb to on-top conditions can also do the trick-if your airplane has the power to climb high enough, and if you're certain of the nearby cloud-top altitudes. Climbing through clouds in icing conditions carries a risk: If you spend too much time at climb angles of attack you could cause ice to form on the undersides of the wings-and aft of any boot- or bleed-air-protected leading edge wing panels. This is a surefire way to kill lift quickly, which is the reason why some manufacturers publish minimum airspeeds for use when climbing in icing conditions.
Often, a 180-degree turn is the best idea. Presumably, you began your flight in ice-free conditions. A return to the areas behind you, then, ought to take you away from danger. What if icing conditions have closed in all around you? A landing at the nearest airport-or a precautionary off-airport landing-is the best move.
The important thing is to have a preconceived idea in your mind as to what you'd do if you inadvertently encountered icing. If you can't come up with a satisfactory plan that has an extremely good chance of success, then the best strategy is not to fly at all.
If you got 'em, pop 'em
Pilots who fly airplanes equipped with inflatable deice boots should inflate those boots as soon as ice forms on wing leading edges. The time-worn advice was to allow a certain amount of ice to form before inflating the boots. That theory was motivated by the belief that cycling the boots too often would cause ice to make a shell-like formation beyond boot-inflation limits. Ice bridging, it was called.
The latest research and scientific conferences, however, have come to the conclusion that ice bridging is a myth. It's true that more ice will shed if more ice is allowed to build on a booted surface. But experts now say there's no reason to believe that ice can continue to form and bridge over leading edges, and leave boots to helplessly pulsate behind an ever-growing sheath of ice.
Those yarns apparently got their start back in the 1930s, when boot inflation pressures were low, inflation times were lengthy, and there were fewer inflatable cuffs within the booted areas.
The most modern boot systems incorporate ice-detecting sensors that automatically initiate inflation cycles.
A decision tree
Avoiding ice starts at the preflight planning stage. Pilots need to take a hard look at themselves, the weather, their airplanes' capabilities, and the terrain before thinking about a flight in situations that could be conducive to icing.
Pilots: If you're not instrument rated, then forget about flying in anything but ice-free conditions, or in weather that's anything less than very good VFR. But should the weather turn ugly, VFR-only pilots must be proficient in the skills and procedures necessary to deal with ATC, and perform climbing or descending turns solely by reference to instruments. Those with instrument ratings should be current and proficient in the basics of instrument flying, and have enough experience in actual instrument conditions to permit some modicum of peace of mind should the need to shoot a tough instrument approach arise.
The weather: Flying in winter fronts is not a good idea in airplanes without certification for flight in known icing. Even with known-ice certification, airplane performance can be crippled by a bout with severe icing.
During the preflight weather briefing, you're looking for above-freezing temperatures at or above any minumum en route altitudes (MEAs). This way, should a descent be necessary you'll lose any ice accretions on the way down. As for cloud tops, they should be low enough that your airplane can top them if a climb out of icing conditions is in order. Ideally, you should have scattered to broken cloud layers along your route of flight, and plenty of holes to allow ice-free climbs and descents to your flight-planned altitudes-and to your destination airport.
Extra caution is called for at night: Icing and other clouds obviously can't be seen as well.
The airplane: For piston-powered airplanes, turbocharging comes in handy in the climbing-to-on-top department. Turbine-powered airplanes seldom have trouble climbing to on-top conditions-as long as the climb is initiated quickly enough. In the clear air above, any ice accumulations that you picked up down below will take some time to sublimate away (it could take hours) but at least you're not collecting any additional ice.
If you're in a piston-powered airplane with a comparatively low horsepower rating, your ability to climb out of ice is seriously compromised. So is your ability to overcome the drag caused by any ice you might pick up. These airplanes, though they may have heated pitot tubes and alternate engine air doors (tools that should be used on any airplane whenever flying in cloud or precipitation within the icing temperature range), just aren't cut out for ice flying. If that's your steed and ice could be a factor, think twice about taking off.
Terrain: Here the concern is flight over mountains and other high terrain. Icing is worse in the air currents over high terrain, and your ability to descend out of icing conditions is severely hampered by high MEAs.
If any of the variables listed above raises any concern, then your preflight decision tree has a shaky limb or two-enough to make you rethink the flight and do the safe thing. You don't need to ground yourself every time clouds pop up in a winter forecast, but you do need to look extra-hard to determine if the trip is really that critical, or if any of the deciding factors raises any level of concern.
Links to other articles on in-flight icing can be found on AOPA Online ( www.aopa.org/pilot/links/links9910.shtml). E-mail the author at [email protected].
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